Though heavily concentrated in California, portions of Florida and the Northeast, higher-cost neighborhoods throughout the country traditionally have depended on the ready availability of “jumbo” mortgages to finance houses.

But with the collapse last year of the private mortgage bond market on Wall Street, home buyers, builders and refinancers who relied on jumbo financing were left with few sources – except at punitively high interest rates and huge down payments.

That's about to change. Major banks are heading into the jumbo segment, originating big loans at affordable rates – not for Wall Street bond traders but for their own investment portfolios.

Bank of America, the country's largest mortgage lender, is rolling out a large program to finance jumbo loans between roughly $730,000 and $1.5 million, with fixed 30-year rates starting in the upper 5 percent range. The loans will be available through the bank's retail network and also through its Countrywide Home Loans subsidiary. After April 27, Countrywide will be rebranded – shedding the name it's had since 1969 – and morph into Bank of America Home Loans. Bank of America acquired Countrywide in 2008.

Barbara Desoer, the bank's head of consumer real estate operations, said there's “a real need” for capital in the jumbo arena, where interest rates last fall sometimes exceeded conventional loan rates by 3 to 5 percentage points – if financing was available at all.

Traditionally, jumbo loans have been defined as any home mortgage whose principal amount exceeded Fannie Mae's or Freddie Mac's statutory high-cost market purchase limit. Most recently that ceiling was $625,500, up from $417,000. But in 2008, Congress temporarily raised the upper limit in high-cost areas for both companies and FHA to $729,750. In the economic stimulus legislation passed by Congress last month, that maximum was extended through Dec. 31 of this year.

Though it will almost immediately become the biggest player in the jumbo loan segment, Bank of America will not be alone. With little fanfare, other financial institutions have become more active.

For example, ING Group, an Amsterdam-based banking and insurance conglomerate, offers jumbos as large as $2 million through its online ING Direct unit. The minimum down payment for an ING Direct jumbo is 25 percent; Bank of America quotes a minimum 20 percent.

ING's jumbos typically are “5/1” and “7/1” hybrids with a fixed interest rate for the first five or seven years, followed by an adjustable rate tied to the LIBOR interbank index for the balance of the 30-year term. Current rates start around 5 percent.

San Diego-based Luxury Loans originates jumbo and “super-jumbo” mortgages of $3 million to $5 million and higher in 50 states for a handful of large commercial banks, who then put them in their investment portfolios.

Victoria Johnson, CEO of Luxury Loans, declined to identify the banks that buy her mega-loans but said their underwriting standards can be rigorous. For example, some investors want proof of substantial cash reserves – at least six months of borrower income – deposited even when down payments are substantial.

In Fort Collins, Colo., Brian Shaver, senior loan officer for 1st City Mortgage Group, originates jumbos through MortgageBase.com, selling them to banks in this country and as far away as Hong Kong. For a loan of $1.5 million to $2.5 million, MortgageBase wants a 40 percent down payment and liquid reserves of 50 percent of the loan amount to qualify for a 4.875 note rate on a 5/1 hybrid.

Johnson of Luxury Loans says she welcomes Bank of America's entry into the mass-market jumbo arena. “We need them,” she says, “because there's been a really serious lack of liquidity at this end of the market” – and that has hurt home prices throughout California as well as parts of the East Coast.

“The more competition,” she says, “the better.” Properly underwritten with solid down payments, large reserves and high credit scores, “jumbos are probably a smart move” for large and small banks that have capacity in their portfolios.

Bottom line: If you've been postponing a purchase, sale or refi because the loan amount you need is too big for Fannie, Freddie or FHA, check out the new, non-Wall Street sources of jumbos.